The present invention relates apparatus for modulating the capacity of a rotary screw compressor. More particularly, the present invention relates an unloading arrangement for a refrigeration screw compressor which is characterized by the disposition of an unloading piston in a cylindrical bore which is remote from the compressor's working chamber but in flow communication with it through a series of non-overlapping unloader ports.
Screw compressor piston unloading arrangements of the type illustrated in U.S. Pat. Nos. 4,042 310; 4,544,333; 4,565,508; and co-pending U.S. patent application No. 07/747,894, which is incorporated herein by reference and which is assigned to the assignee of the present invention, are unloading arrangements which employ an axially movable or rotatable unloading piston disposed within a cylindrical bore remote from the compressor's working chamber The bore communicates with the working chamber through a series of axially arranged unloader ports and is additionally in flow communication with a portion of the compressor which is at compressor suction pressure in operation.
When the unloading piston in such arrangements is positioned within the unloader bore so as to completely interrupt communication of the bore with the compressor's working chamber through the unloader ports the compressor operates fully loaded because the compression pockets defined in the working chamber are prevented from unloading to suction through the unloader ports and bore The unloading piston is moved axially or is rotated within the bore to fully or partially cover or uncover the unloader ports in a sequential manner thereby providing for the selective and variable communication of the compression pockets within the working chamber back to suction for the purpose of unloading the compressor.
With respect to the arrangement in assignee's copending patent application and referring to FIGS. 1, 2 and 3 herein which are, respectively, FIGS. 1, 5 and 6 in assignee's co.pending patent application. FIG. 1 is a partial cross-sectional side view of a screw compressor illustrating piston unloader apparatus associated with the male rotor of a screw compressor with the unloader piston in the full unload position. Compressor 10 is comprised of a rotor housing 12 and bearing housing 14. A motor 16, male rotor 18 and female rotor (not shown) are disposed in the rotor housing. Shaft 22 extends from the male rotor and motor rotor 24 is mounted thereon.
Suction gas enters rotor housing 12 through the suction end 26 of the compressor and passes through a suction strainer (not shown) prior to passing through and around motor 16 in a manner which cools the motor. In this regard, suction gas passing through and around motor 16 passes out of motor-rotor housing gap 28, rotor-stator gap 30 and into suction area 32 within the rotor housing. The gas next passes from suction area 32, through suction port 34 and into the working chamber 36 where it is enveloped in a chevron shaped compression pocket defined by the wall of the working chamber and the intermeshed lobes of male rotor 18 and the female rotor.
As the male and female rotors rotate, the pocket in which the suction gas is initially enveloped is closed off from suction port 34 and is circumferentially displaced toward high pressure end wall 38 of the compressor s working chamber. As such displacement occurs, the volume of the pocket is reduced and the gas contained therein is compressed until such time as the pocket opens to discharge port 40.
Rotor housing 12 defines a cylindrical bore 50 which is in flow communication with suction port 34 or some other area of the compressor or system in which the compressor is employed which is at suction pressure. Rotor housing 12 also defines a series of ports 52 which communicate between bore 50 and working chamber 36. Disposed in bore 50 is an unloader piston 54 which includes a control portion 56 disposed in a chamber 58 defined by the bearing housing. Unloader piston 54 is axially positionable within bore 50 so as to provide for the selective occlusion of ports 52.
Ports 52 are generally elongated axially running curvilinear slots defined in the wall of working chamber 36 of the rotor housing. Ports 52 overlap each other in the axial sense so as to provide, through their interaction with unloader piston 54, for an essentially continuous unloading path from the male rotor portion of the working chamber into bore 50. The length of that path and, therefore, the capacity of the compressor is determined by the position of piston 54 within bore 50 and the extent to which ports 52 are occluded by the unloader piston.
Piston 54 is preferably hydraulically actuated with chamber 58 being in flow communication with a source of pressurized fluid, such as the lubricant employed within the compressor, through passage 62 in which a solenoid operated load valve 64 is disposed. Chamber 58 is likewise in flow communication with passage 66 in which a solenoid operated unload valve 68 is disposed.
By porting oil which is at discharge pressure through load valve 64 with unload valve 68 closed, piston 54 is caused to move axially toward suction end 26 of the compressor thereby further loading the compressor through the occlusion of additional ones of ports 52 or a portion thereof. Contrarily, the opening of unload solenoid 68, with load valve 64 closed, places passage 66 in flow communication with a portion of compressor 10 which is at less than suction pressure thereby permitting discharge pressure gas, which is communicated through passage 70 into chamber 58 to act on the side of control portion 56 of piston 54 opposite from the side operated on by a pressurized fluid. This causes piston 54 to move away from the suction end of the compressor which causes the compressor to unload as additional ones or parts of unloader ports 52 are opened.
As is noted in assignee's co.pending U.S. Pat. application, unloading ports 52 effectively overlap each other, in the axial sense, so as to provide an essentially continuous unloading path from the male rotor portion of the working chamber into the unloader bore and for essentially continuous compressor unloading along that path. This essentially continuous unloading path results from the overlap of the unloading ports. The unloading piston has an essentially flat end face so that as soon as unloader 54 is moved to completely occlude or uncover a first unloader port any further movement of it will begin to occlude or uncover the next unloader port in its direction of travel. It is the interaction of this type of unloader piston with the overlapping unloader ports which permits the continuous unloading of the compressor.
It has been determined that the use of elongated overlapping unloader ports, such as those described in assignee's co-pending patent application with the unloader piston taught therein, while allowing for the essentially continuous unloading of a screw compressor, brings with it certain disadvantages in the form of a less formidable seal against leakage between adjacent ones of the unloader ports around the unloader piston. Such leakage together with the relatively large clearance volume of the elongated unloader ports, results in compressor efficiencies and capacities which can be improved upon.
Such improved efficiencies and capacities are necessary to make screw compressors with their very distinct advantage of being able to be unloaded over a continuous operating range, economically competitive with the other, less expensive compressor designs against which they must compete in lower capacity ranges. Therefore, the need to improve upon the unloading arrangement associated with the male rotor of the screw compressor in assignee's co-pending patent application and screw compressor unloaders in the general sense to achieve improved compressor efficiency and increased capacity was identified.